package com.shawn.algorithm.string;

public class KMP {

	public static void main(String[] args) {
		final String haystack = "AAAAABAAABA";        //This is the full string
		final String needle = "AAAA";                //This is the substring that we want to find
		KMPmatcher(haystack, needle);
	}

	// find the starting index in string haystack[] that matches the search word P[]
	public static void KMPmatcher(final String haystack, final String needle) {
		final int m = haystack.length();
		final int n = needle.length();
		final int[] pi = computePrefixFunction(needle);
		int q = 0;
		for (int i = 0; i < m; i++) {
			while (q > 0 && haystack.charAt(i) != needle.charAt(q)) {
				q = pi[q - 1];
			}

			if (haystack.charAt(i) == needle.charAt(q)) {
				q++;
			}

			if (q == n) {
				System.out.println("Pattern starts: " + (i + 1 - n));
				q = pi[q - 1];
			}
		}
	}

	// return the prefix function
	private static int[] computePrefixFunction(final String P) {
		final int n = P.length();
		final int[] pi = new int[n];
		pi[0] = 0;
		int q = 0;
		for (int i = 1; i < n; i++) {
			while (q > 0 && P.charAt(q) != P.charAt(i)) {
				q = pi[q - 1];
			}

			if (P.charAt(q) == P.charAt(i)) {
				q++;
			}

			pi[i] = q;

		}
		return pi;
	}
}
